Virtual machines (VMs) running in computer systems may access memory resources encapsulated within virtual volumes (VVOLs) that are managed and exported by a tiered storage system. VVOLs are VM-centric logical aggregations of physical storage units included in the tiered storage system. Detailed descriptions of VVOLs are set forth in U.S. patent application Ser. No. 13/219,358, filed Aug. 26, 2011 and entitled “Object Storage System,” the entire contents of which are incorporated by reference herein. To optimally utilize the available physical storage units, each VVOL may be assigned to a different “storage tier,” where each storage tier includes types of physical storage units with distinct performance characteristics. For example, each storage tier may represent a different type of drive, SSD, SAS, SATA, SCSI, rotational speed, and/or RAID level.
In an attempt to increase the performance of the computer system, some tiered storage systems observe the input/output performance of the physical storage units and transparently move data from one storage tier to another storage tier in a process known as “auto-tiering.” In such a tiered storage system, if the tiered storage system observes increased I/O activity involving a particular logical storage volume, then the tiered storage system may promote the logical storage volume to a higher performance tier. While such a reactive auto-tiering algorithm may improve performance in non-virtualized environments, past I/O activity is not typically a good indication of future I/O activity across the VVOL life cycle and, consequently, the reactive auto-tiering algorithm may not be effective in virtualized environments.
More specifically, during the VVOL life cycle, different VM operations typically generate numerous I/O operations for a relatively short period of time and then the number of I/O operations drops dramatically. For example, when a VM is suspended there will be a high number of I/O operations associated with copying the memory to the disk. If a storage tiering system with a reactive auto-tiering algorithm observes the high number of I/O operations, then the storage tiering system may move the VVOL corresponding to the VM to a storage tier with high I/O performance. However, after the suspend operation is completed, the suspended VM will not perform any I/O operations on the VVOL, and the high performance physical memory resources will be wasted. Further, if the time required to move the VVOL exceeds the time required to perform the suspend operation, then performing the tier movement operation reduces the overall performance of the computer system. Consequently, there is a need for an improved storage tiering mechanism in virtualized environments.